The present invention relates to a multimode wireless terminal (or a portable telephone) capable of being connected to wireless networks of a plurality of kinds, which are different in wireless communication scheme. In particular, the present invention relates to a multimode wireless terminal and a wireless transmitter-receiver unit suitable for handover conducted between different wireless network systems.
As mobile communication (portable telephone) service networks, various kinds of wireless network systems differing in communication protocol are in operation. For example, wireless networks of the GSM (Global Systems for Mobile communications) are called second generation portable telephone wireless network. Communication service in the GSM wireless networks was started in the 900 MHz band. As the number of subscribers increases thereafter, however, the frequency band is expanded. At the present time, multiband portable telephones that can be used in the 1800 MHz band and the 1900 MHz band as well are spread. As the second generation portable telephone wireless network, PDC, PHS, CDMA of IS-95, cdmaOne (registered trademark) conforming to 3GPP2, and MC-CDMA are known besides the GSM.
On the other hand, as for the WCDMA (Wideband Code Division Multiple Access) scheme proposed as third generation portable telephone wireless networks, communication service in Japanese territory was started in 2002, and overseas communication service in Europe and so on is under contemplation.
In a communication environment in which telephone service networks of a plurality of kinds that are different in wireless communication protocol thus coexist, a wireless terminal (portable telephone) that can be selectively connected to a plurality of telephone service networks with the same terminal becomes convenient. A compound wireless terminal of this kind is typically called multimode type, dual mode type, or multisystem type. By using the multimode wireless terminal, it becomes possible to enjoy telephone service in an optimum communication mode according to the current position of the wireless terminal.
Furthermore, by using a network configuration in which first and second mobile gateway switches that are different in wireless communication protocol are connected to a PSTN (Public Switched Telephone Network) via mobile inter-system gateway switches, it becomes possible to, for example, hand over a multimode wireless terminal that is communicating with another terminal via a wireless base station accommodated in the first mobile gateway switch to a wireless base station using a different protocol accommodated in the second mobile gateway switch without disconnecting a call. Such base station changeover between wireless networks differing in communication protocol is called intersystem handover. By implementing seamless intersystem handover without disconnecting operation of the call connection and communication application while a wireless terminal is conducting communication, communication service using optimum transmission rates and optimum lines according to the application becomes possible.
As a conventional technique concerning the multimode wireless terminal, for example, a portable wireless machine is proposed in JP-A-8-186516. In the portable wireless machine, wireless units of a plurality of kinds respectively associated with wireless network systems and a baseband processor unit shared by the wireless network systems are included, and connection between a wireless unit and the baseband processor unit is changed over according to user's operation.
Furthermore, as a conventional technique concerning the intersystem handover, for example, a communication scheme changeover method is proposed in JP-A-2002-77965. When an intersystem handover request has occurred, the degree of channel margin in a base station of a changeover destination wireless network is calculated and it is determined on the basis of the degree of channel margin whether handover is possible, according to the communication scheme changeover method. At the time of handing over from the third generation wireless network such as the WCDMA to the second generation wireless network such as the GSM, control channel information of the second generation wireless network is provided for the wireless terminal by a down-link control channel of the third generation wireless network. This is proposed in JP-A-2002-535902.